The present disclosure provides a method for fabricating a thin film transistor substrate, which includes: sequentially depositing a light shielding layer pattern, a buffer layer, an active layer pattern, a gate insulating layer, and a gate layer; wet etching the gate layer to form a gate layer pattern with a photoresist; stripping off the photoresist; forming a protective layer covering the gate layer pattern; etching the gate insulating layer to form a gate insulating layer pattern; and metalizing a non-channel region of the active layer pattern. This method can ensure that an orthographic projection of the gate layer pattern on the substrate completely coincides with that of the gate insulating pattern. Therefore, the entire active layer pattern is regulated by the gate layer pattern, thereby improving a turn-on current of a thin film transistor.